The present invention relates to image reading devices for use in, for example, copying machines and facsimiles, and in particular, to an image reading device which is adapted to continuously read large amounts of documents using a light source such as a fluorescent lamp whose light quantity varies depending on an ambient temperature and other factors.
Conventionally, light quantity correction of a light source for illuminating a document has been executed at a constant frequency, i.e., at constant intervals, even if a copying mode (eg. a high image quality mode and a high production mode) differs.
For example, if the light quantity correcting frequency is adjusted to a high image quality mode, the light quantity correction operation is performed frequently such that the image quality is maintained high. However, such frequent light quantity corrections require comparatively much time. Accordingly, disadvantageously, a productivity in the high production mode is reduced. Conversely, if the light quantity correcting frequency is adjusted to the high production mode, there is a problem that the image quality in the high image quality mode may deteriorate.
Also, there is known a device which absorbs fluctuation in light quantity of a light source by adjusting the light quantity of the light source every time a document page is read (Japanese Patent Publication No. HEI 5-30102).
However, performing a light quantity adjustment so frequently leads to a problem that a warm-up time and a read time for a first copy become long, causing a reduction in productivity.
There is a further concern that the fluctuation in light quantity would be conversely increased by frequently executing the light quantity adjustment. For example, if the light quantity considerably decreases during a continuous read operation and thus the decrease in light quantity is corrected, a light quantity adjustment value, i.e., a lighting control value may considerably increase. Accordingly, upon lighting the light source again after an environmental temperature has decreased after completion of the above continuous read operation, there may be a saturation in light quantity of the light source.